RoGERS. | Minerals of the Galena-Joplin District. 451 
different trisoctahedra evidently serve as twinning planes. 
From the first angle observed, a jb, it has been calculated that 
the interfacial angle between the octahedron and the trisocta- 
hedron serving as the twinning planeis31° 28’. The theoret- 
ical for the form {11-11-1} 11lis 31° 352’. A slight difference 
in the angle makes a great difference in the symbol, so that 
the symbol given may not be the correct one. Sometimes the 
bands are apparently parallel to the cleavage lines. If such 
is really the case the twinning plane is the rhombic dodeca- 
hedron. But the latter being a plane of digonal symmetry 
is not a possible plane of symmetry in holohedral crystals. 
It is then very probable that in this case a trigonal trisocta- 
hedron is the twinning plane. Occasionally the bands in the 
two different directions indicated are both present on the 
same cleavage surface, and these are often very much con- 
fused, so that the surface is broken up into a multitude of 
minute planes, which apparently bear no relation to each 
other. On other crystals or parts of crystals the laminz are 
undulating, the slight dihedral angles not being sharp or 
well defined. Most of the twinned crystals show no effects 
of pressure or of distortion of any kind. 
Lamellar twinning in galena is reported by Cross’ from 
the ‘‘ Minnie Moore’’ mine, Bellevue, Idaho, and by Hobbs,’ 
from Mineral Point, Highland, Yellowstone, and Platteville, 
Wis. 
Besides the two common forms named above, the cube and 
octahedron, another was observed on a singlecrystal. This, 
an apparent cubic face, on closer examination proved to be a 
vicinal tetragonal trisoctahedron. What apparently is a 
cubic face in reality is broken up into several different faces 
with very slight reentrant and salient angles, their edges be- 
ing parallel to the cleavage lines, or intersection edges of the 
cubic faces. This may be due to twinning, as twinning 
lamine are seen on the broken surface of the crystal. But here 
etch-figures give valuable aid. (What I take for etch-figures 
may be contemporary with the formation of the crystal, and 
hence rather to be classed as parallel growths or growth 
1. Cross: Proc. Col. Sci. Soc., vol. 2, pp. 171-174, 1887. 
2. Hobbs: Zeit. f. Kryst. u. Min., vol. 65, pp. 23-265, 1895. Id., Bull. Univ. Wisconsin, 
Science Series, vol. 1, pp. 125-128, 1895. 
